1. Field of the Invention
The present invention relates to a screw and a screw fastening structure, the surface of which is provided with the treatment of plating and is provided with the treatment of a trivalent chromium-containing and hexavalent chromium-free chemical conversion coating on top of the layer thereof.
2. Description of the Related Art
Conventionally, a surface is provided with the treatment of plating and also provided with a chromate conversion coating thereon for preventing screws from corroding. Among the chromate conversion coatings, especially a hexavalent chromate conversion coating presents high moisture content, comparatively soft and has excellent corrosion resistance because of a self-healing action which is a characteristic feature thereof. However, since a hexavalent chromium is hazardous to human bodies, and results in the environmental pollution and the like, the use of a hexavalent chromium is substantially restricted in the world. Consequently, it has been under review to replace the conventional hexavalent chromium-containing chromate treatment with the hexavalent chromium-free alternative treatment. A generally acceptable alternative is chemical conversion treatment containing trivalent chromium compounds. A patent document disclosing such alternatives is for example Japanese Translation of Unexamined PCT Appln. No. 2000-509434.
However, a screw provided with the treatment of general trivalent chromium-containing chemical conversion coatings has a problem that a torque characteristic drops in fastening and, especially, a loosening torque decreases greatly because a chromate conversion coating of trivalent chromium is harder than a hexavalent chromate conversion coating. A considerable difference is generated between a torque characteristic in the treatment of chemical conversion coatings of trivalent chromium and hexavalent chromate treatment especially in the case of the application of a bolt to a high axial force range among other screws, for example a crank pulley bolt and the like. To be more specific, the loosening torque of the bolt by the treatment of chemical conversion coatings of trivalent chromium declines by 10% to 30% compared to the bolt by the hexavalent chromate treatment. This is nothing less than a decrease in a fastening force of the bolt applied for fixing a structure member. Therefore, functions guaranteed by a bolt of the conventional hexavalent chromate treatment could not be satisfied. In addition, a general treatment of chemical conversion coatings of trivalent chromium has adopted a multilayer structure in which other metal compositions are contained in the coating, a SiO2-containing layer or coating is added on the surface, and the like, in order to compensate corrosion resistance properties originally inferior to hexavalent chromium. Consequently, a chemical conversion coating of trivalent chromium becomes harder and exerts such a great influence to the torque characteristic of the screw as especially a loosening characteristic is made deteriorating and the like.